a) Field of the Invention
The invention is directed to an arrangement for generating extreme ultraviolet radiation based on an electrically operated gas discharge with a discharge chamber, which has a discharge area for a gas discharge for forming a plasma that emits the radiation, a first disk-shaped electrode and a second disk-shaped electrode, at least one of which is mounted so as to be rotatable, an energy beam source for providing an energy beam, and a high-voltage power supply connected to the electrodes for generating high-voltage pulses.
b) Description of the Related Art
The invention is applied as a light source of short-wavelength radiation, preferably for EUV lithography in the fabrication of integrated circuits. However, it can also be used for incoherent light sources in other spectral ranges from soft x-ray to infrared.
Radiation sources which are based on plasmas generated by gas discharge and which rely on various concepts have already been described many times. The principle common to these arrangements consists in that a pulsed high-current discharge is ignited in a gas of determinate density, and a very hot and dense plasma is generated locally as a result of the dissipated power in the ionized gas.
Since the heated plasma emits not only radiation but also high-energy ions, debris mitigation tools, as they are called, are provided particularly for protecting collector optics which make the radiation emitted more or less homogeneously in all spatial directions available for applications such as wafer exposure.
When gases are used in the debris mitigation tools for deceleration of the high-energy ions, the required gas pressure also leads to an increase in the background pressure in the region of the electrode arrangement. As soon as the breakdown voltage decreases as a result below the operating voltage of the gas discharge, avalanche ionization causes parasitic discharges so that less energy can be dissipated in the actual gas discharge.
Therefore, it is necessary to charge the electrodes to the operating voltage of the discharge within a shorter period of time than that required by the avalanche ionization for a parasitic breakdown. This means that the recharging time τ=π√{square root over (LC)} from the final capacitor—usually constructed as a capacitor battery—of the high-voltage power supply to the electrode must be shortened. Since the stored energy E depends on the capacitance of the capacitor and on the applied voltage, as expressed by
      E    =                  1        2            ⁢              CU        2              ,the capacitance C cannot be reduced to any desired value when a definite energy must be provided for the gas discharge and the voltage must not be driven up extremely high. Therefore, the inductance L of the discharge circuit, which is made up of the line inductance, the self-inductances of the electrode arrangement and the capacitance C, must be maintained as low as possible.
In a previously known device according to WO 2005/025280 A2 which uses rotating electrodes that dip into a vessel containing molten metal, an additional metal screen which is arranged as nearly as possible between the electrodes to prevent the penetration of a magnetic field through eddy currents during the discharge is suggested as a low-inductance circuit. However, owing to the fact that the current pulse is conveyed to the electrodes by the molten metal in that the capacitors needed for storing the electrical energy for plasma generation are electrically connected to the molten metal in the vessels by means of a plurality of metal pins or strips which are embedded in a vacuum-tight manner in insulators, a high inductance results because of the required current supply to the electrodes.
Therefore, there is a need to reduce the time required for charging the electrodes by reducing the inductance of the discharge circuit.
This need is met in an arrangement for generating extreme ultraviolet radiation based on an electrically operated gas discharge of the type mentioned above in that the high-voltage power supply has a capacitor battery comprising capacitor elements which are arranged along a ring concentric to the axis of rotation with a ring plane directed parallel to the disk surface, and electrical connections are guided to the disk surfaces from the capacitor elements along a ring concentric to the axis of rotation.
Particularly advisable and advantageous constructions and further developments of the arrangement according to the invention are indicated in the dependent claims.
Due to the contact over a large outer radius and the resulting reduction in self-inductance, the invention makes it possible to operate with a higher gas pressure because the electrodes can be charged faster.
In contrast to WO 2005/025280, in which the inductance is determined by a discharge circuit similar to a double line, the substantially lower inductance of the discharge circuit in the invention corresponds to that of a toroidal coil with one winding so that the energy can be transferred from the final capacitor battery in the high-voltage power supply to the electrodes in less than 1 μs and can be made usable for the gas discharge.
It is possible to recharge the energy from a capacitor battery arranged upstream to this final capacitor battery by means of magnetic pulse compression. This means that saturable inductances having a very high relative permeability (μr≈12000) are used for pulse shortening and therefore initially cause a substantial delay in the recharging of the electrical energy. In the presence of a magnetic field, however, the relative permeability is sharply reduced (μr≈2) and a fast recharging of the energy is possible so that with a suitable layout of the capacitors and the saturable inductances the electric pulse can be shortened by more than a factor of 10.
The rotary electrode arrangement according to the invention in which the disk-shaped electrodes are rigidly connected at a distance from one another to a rotatably mounted shaft allows current pulses to be supplied to the electrodes without wear and, above all, with low inductance.
Therefore, the invention can be constructed in such a way that the electrical connections leading to the disk surfaces have contact elements which are oriented coaxial to the axis of rotation and which are immersed in ring-shaped baths of molten metal which are electrically separated from one another and which communicate with the capacitor elements of the high-voltage power supply.
In a preferred constructional variant of the invention, one electrode has, as contact element, a plurality of individual contacts which are electrically connected to the disk surface of the one electrode along a ring and are guided through openings in the other electrode so as to be electrically insulated, and the contact element of the other electrode is constructed as a closed cylinder ring placed on the disk surface.
As an alternative to the construction mentioned above, the electrical connections can also be guided from the capacitor elements to the disk surfaces via sliding contacts.
The capacitor battery can be arranged inside or outside the discharge chamber. In the latter arrangement, the discharge chamber has vacuum feedthroughs through which the electrical connections are guided.
Also, the shaft to which the electrodes are connected can be guided into the discharge chamber via a vacuum feedthrough and driven by driving means arranged outside the vacuum chamber.
It is advantageous when the shaft has at least one bore hole in longitudinal direction for moving coolant to the electrodes. Coolant is guided into the electrodes through cooling channels at a pressure between 1 bar and 30 bar.
It is also possible for each disk-shaped electrode to be rigidly connected to a respective rotatably mounted shaft, these shafts having a common axis of rotation and identical rotational speeds so that the position of the electrodes relative to one another does not change during the rotation. This is important because it is also necessary in this construction to guide the contact of one electrode through corresponding openings in the other electrode. This construction of the invention has advantages above all when the coolant is supplied to the electrodes via the shafts so that a shaft is available for supplying coolant to each electrode.
Further, the invention can be constructed in such a way that an injection device is directed to the discharge area. At a repetition rate corresponding to the frequency of the gas discharge, this injection device provides a series of individual volumes of an emitter material serving to generate radiation, and the individual volumes are limited in amount so that the emitter material which is injected into the discharge area at a distance from the electrodes is entirely in the gas phase after the discharge. The energy beam supplied by the energy beam source is directed synchronous in time with the frequency of the gas discharge to a plasma generation site in the discharge area which is provided at a distance from the electrodes and in which the individual volumes arrive so as to be ionized successively by the energy beam.
The surface erosion of the electrodes can be countered when at least one of the electrodes has in the edge area a layer of a continuously applied molten metal and the edge area has at least one receiving area which extends circumferentially in a closed manner along the edge of the electrode on the electrode surface and which is constructed so as to be wetting for the molten metal and to which a device for introducing the metal is directed.
The invention will be described more fully in the following with reference to the schematic drawings.